SMTP and FTP

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Chapter 26
SMTPandFTP
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• Email is used for sending a single message that
  includes text, voice, video, or graphics to one
  or more recipients.
• Simple Mail Transfer Protocol (SMTP).
• Sending Mail
• User creates mail that looks very similar to
  postal mail.
• Envelope sender and receiver address
• Message
• Header Define the sender, receiver, subject of
  the message, ...
• Bodycontains the actual information to be read
  by the recipient.
• Receiving mail
• Email system periodically checks the mailboxes.
• If a user has mail, it informs the user with a
  notice
• If the user is ready to read the mail, a list is
  displayed in which each line contains a summary
  of the information about a particular message in
  the mailbox. Summary contains the sender mail
  address, the subject, and time the mail was sent
  or received.

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Figure 26.1 Format of an email
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Figure 26.2 Email address

• Mail handling system must use an addressing
  system with unique addresses.
• Local part Name of a special file, called the
  user mailbox, where all the mail received for a
  user is stored for retrieval by the user agent.
• Domain Name
• Organization usually selects one or more hosts to
  receive and send email they are sometimes called
  mail exchangers.
• Domain name assigned to each mail exchanger
  either comes from the DNS database or is a
  logical name (e.g., the name of the organization).

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Figure 26.3 User agent

• First component of an electronic mail system is
  the user agent (UA) Mail Reader.
• Service provided by a user agent
• User agent is a software package (program) that
  composes, reads, replies to, and forward
  messages.
• It also handles mailboxes.
• Composing messages
• Responsible for composing the email msg to be
  sent out.
• Some of them provide a template and some provide
  with built-in editor OR else user can type in an
  editor and paste into the user agent.

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• Reading messages
• User agent is used to read incoming messages.
• When a user invokes a user agent, it first checks
  the mail in the incoming mailbox. A one-line
  summary number, flag field indicating if the
  mail is new OR already read but not replied to OR
  read and replied to OR , size of message,
  sender, subject if subject line in msg is not
  empty of each received mail is shown.
• Replying to messages
• After the reading a message, user can use the
  user agent to reply to a message.
• User agent allows the user to reply to the
  original sender or to reply to all recipients of
  the message.
• Reply contains the original message (for quick
  reference) and new message.
• Forwarding messages Can forward with or without
  extra comments.
• Handling Mailboxes
• Inbox Keeps all received emails until they are
  deleted by the user.
• Outbox Keeps all sent emails until the user
  deletes them.

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User Agent Types

• Command-Driven
• Normally accepts a one-character command from the
  keyboard to perform its task.
• mail, pine, elm,
• GUI-Based
• Contain GUI components that allow the user to
  interact with the software by using both the
  keyboard and the mouse.
• Eudora, Microsofts Outlook, Netscape,
• SMTP is a simple mail transfer protocol
• Can send messages only in 7-bit ASCII format.
• Cannot be used to send binary files or video or
  audio.
• Cannot support few languages that are not
  supported by ASCII

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Figure 26.4 MIME

• Multipurpose Internet Mail Extensions (MIME)
• Supplementary protocol that allows non-ASCII data
  to be sent through SMTP.
• Only an extension to SMTP.
• MIME transforms non-ASCII data at the sender side
  to ASCII data and delivers them to client SMTP to
  sent through the Internet. Reverse is done at the
  receiver side.

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Figure 26.5 MIME header

• MIME defines five headers that can be added to
  original SMTP header
• MIME-Version
• Content-Type Type of data used in the body has
  subtypes separated with slashes.
• Content-Transfer-Encoding
• Content-Id Uniquely identifies the whole message
  in a multiple-message environment.
• Content-Description Whether the body is image,
  audio, or video.

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Content Types

• Text
• 7-bit ASCII no transformation by MIME
• Multipart
• Multiple, independent parts.
• Needs to define the boundary between each part,
  as a parameter.
• Boundary is terminated using two hyphens.
• Mixed Present the parts in exact order as they
  are in message.
• Parallel Order of parts is unimportant.
• Digest Same as mixed but default type/subtype is
  message/RFC822
• Alternative Same message is repeated using
  different formats.

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• Message
• Body is itself a whole mail message, a part of a
  mail message, or a pointer to a message.
• Subtypes
• RFC822 Used if the body is encapsulating another
  message (including header and the body).
• Partial Used if the original message has been
  fragmented into different mail messages and this
  message is one of the fragments.
• Fragments must be reassembled at the destination
  by MIME.
• Three parameters must be added id, number, total
• Id identifies the message and is present in all
  the fragments.
• Number sequence order of the fragment
• Total number of fragments that comprise the
  original message
• External-body.
• Body does not contain the actual message but is
  only a reference (pointer) to the original
  message.

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• Image
• Original message is a stationary image,
  indicating that there is no animation.
• Joint Photographic Experts Group (JPEG)
• Graphics Interchange Format (GIF)
• Video
• Original message is a time-varying image
  (animation).
• Motion Picture Experts Group (MPEG)
• If the animated image contains sounds, it must be
  sent separately using the audio content type.
• Audio
• Original message is a sound.
• Only subtype is basic, which uses 8-KHz standard
  audio data.
• Application
• Original message is a type of data not previously
  defined.
• Octet-stream is used when the data must be
  interpreted as a sequence of 8-bit bytes (binary
  file).
• PostScript is used when the data are in Adobe
  PostScript format.

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Table 26.1 Data types and subtypes in MIME
Type Subtype Description
Text Plain Unformatted text 7-bit ASCII
Multipart Mixed Body contains ordered parts of different data types
Multipart Parallel Same as above, but no order
Multipart Digest Similar to mixed, but the default is message/RFC822
Multipart Alternative Parts are different versions of the same message
Message RFC822 Body is an encapsulated message
Message Partial Body is a fragment of a bigger message
Message Ext. Body Body is a reference to another message
Image JPEG Image is in JPEG
Image GIF Video is in GIF format
Video MPEG Video is in MPEG format
Audio Basic Single-channel encoding of voice at 8 KHz
Application PostScript Adobe PostScript
Application Octet-Stream General binary data (8-bit bytes)
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Content-Transfer-Encoding

• Method to encode the messages into 0s and 1s for
  transport.
• 7bit
• 7-bit ASCII encoding
• Length of line should not exceed 1000 characters
• 8bit NOT Recommended
• 8-bit encoding can send non-ASCII characters.
• Length of line should not exceed 1000 characters
• MIME does not do any encoding here
• SMTP must be able to transfer 8-bit non-ASCII
  characters.
• Binary NOT Recommended
• 8-bit encoding
• Non-ASCII characters can be sent, and the length
  of the line can exceed 1000 characters.
• MIME does not do any encoding here SMTP must be
  able to transfer binary data.
• Base64
• Solution for sending data made of bytes when the
  highest bit is not necessarily zero.
• Base64 transforms this type of data to printable
  characters, which can then be sent as ASCII
  characters or any type of character set supported
  by underlying mail transfer mechanism.

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Table 26.2 Content-transfer encoding
Category Description
Type ASCII characters and short lines
7bit Non-ASCII characters and short lines
8bit Non-ASCII characters with unlimited-length lines
Binary 6-bit blocks of data are encoded into 8-bit ASCII characters
Base64 Non-ASCII characters are encoded as an equal sign followed by an ASCII code
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Figure 26. 6 Base64
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Table 26.3 Base64 encoding table
Value Code Value Code Value Code Value Code Value Code Value Code
0 A 11 L 22 W 33 h 44 s 55 3
1 B 12 M 23 X 34 i 45 t 56 4
2 C 13 N 24 Y 35 j 46 u 57 5
3 D 14 O 25 Z 36 k 47 v 58 6
4 E 15 P 26 a 37 l 48 w 59 7
5 F 16 Q 27 b 38 m 49 x 60 8
6 G 17 R 28 c 39 n 50 y 61 9
7 H 18 S 29 d 40 o 51 z 62
8 I 19 T 30 e 41 p 52 0 63 /
9 J 20 U 31 f 42 q 53 1
10 K 21 V 32 g 43 r 54 2
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Figure 26.7 Quoted-printable

• Quoted-Printable
• Base64 is a redundant encoding scheme that is,
  24 bits becomes four characters and eventually is
  sent as 32 bits.
• If the data consist mostly of ASCII characters
  with a small non-ASCII portion, we can use
  quoted-printable encoding.
• If a character is
• ASCII, it is sent as it is.
• Non-ASCII, sent as three characters. First
  character is the equals sign (). Next two
  characters are the hex of the byte.

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Figure 26.8 MTA client and server

• A mail transfer agent or MTA (also called a mail
  server, or a mail exchange server is a computer
  program or software agent which transfers
  electronic mail messages from one computer to
  another. The MTA works behind the scenes, while
  the user usually interacts with another program,
  the mail user agent (MUA), which contacts an MTA
  for actual delivery of the mail.

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• Typical MTAs
• Sendmail
• Postfix
• Exim
• QMail

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Figure 26.9 Commands and responses

• SMTP uses commands and responses to transfer
  messages between an MTA client and an MTA server.
• Each command or reply is terminated by a
  two-character (carriage return and line feed)
  end-of-line token.
• Commands
• Commands are sent from the client to the server.
• Consists of a keyword followed by zero and more
  arguments.
• SMTP defines 14 commands.
• Responses
• Responses are sent from server to client.
• Response is a three-digit code that may be
  followed by additional textual information.

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Mail Transfer Phases

• Process of transferring a mail message occurs in
  three phases
• Connection establishment
• After a client has made a TCP connection to the
  well-known port 25, the SMTP server starts the
  connection phase.
• Message transfer
• After connection establishment, a single message
  between a sender and one or more recipients can
  be exchanged.
• Connection termination
• After the message is transferred successfully,
  the client terminates the connection.

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Mail Delivery

• Delivery of email from sender to receiver
  consists of three stages
• 1st stage
• Email goes from user agent to local server
• Mail does not go directly to remote server
  because the remote server may not be available at
  all times.
• Mail is stored in the local server until it can
  be sent.
• 2nd stage
• Email is relayed by local server, which now acts
  as the SMTP client, to the remote server, which
  is the SMTP server.
• Email is delivered to remote server and not to
  the remote user agent Email is stored in mailbox
  of the user for later retrieval..
• Email server runs the SMTP server program.
• 3rd stage
• Remote user agent uses a mail access protocol
  such as POP3 or IMAP4 to access the mailbox and
  obtain the mail.

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Figure 26.10 Email delivery
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Mail access protocols

• SMTP is a push protocol it pushes the message
  from the sender to the receiver even if the
  receiver does not want it.
• 3rd stage needs a pull protocol the operation
  must start with the recipient.
• Mail must stay in the mail server mailbox until
  the recipient retrieves it.
• POP3 Post Office Protocol
• Client POP3 software is installed on the
  recipient computer.
• Server POP3 software is installed on the mail
  server.
• Client (user agent) opens a connection with the
  server on TCP port 110.
• It then sends its user name and password to
  access the mailbox.
• User can then list and receive mail messages, one
  by one.

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• POP3 modes
• Delete mode Mail is deleted from the mailbox
  after each retrieval.
• Keep mode Mail remains in the mailbox after
  retrieval.
• IMAP4 (Internet Mail Access Protocol)
• POP3 assumes that each time a client accesses the
  server, the whole mailbox will be cleared out
  (transferred).
• POP3 does not allow user to organize mail on the
  server user cannot have different folders on the
  server POP3 does not allow the user to partially
  check the contents of the mail before
  downloading.
• User can check the email header prior to
  downloading
• User can search the contents of email for a
  specific string of characters prior to
  downloading
• User can partially download email.
• User can create, delete, or rename mailboxes on
  mail server
• User can create a hierarchy of mailboxes in a
  folder for email storage.
• Web-Based Email Hotmail or Yahoo
• Mail transfer from client browser to mail server
  is via HTTP
• SMTP is used for mail transfer from sending mail
  server to receiving mail server.
• Message from receiving mail server to browser is
  via HTTP

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Figure 26.11 POP3
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File Transfer Protocol

• Copying a file from one host to another
• Problems in FTP
• Two systems may use different file name
  conventions
• Two systems may have different ways to represent
  text and data.
• Two systems may have different directory
  structures.
• FTP establishes two connections between client
  and server.
• One connection is used for data transfer Port
  20
• Other connection for control information
  (commands and responses) Port 21
• Separation of commands and data transfer makes
  FTP more efficient. Commands are very simple.

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Figure 26.12 FTP

• Client has three components
• User interface, Client control process, client
  data transfer process.
• Server has two components
• Server control process, server data transfer
  process
• Control connection is made between control
  processes Control connection is maintained
  during entire interactive FTP session.
• Data connection is made between data transfer
  processes Data connection is opened then
  closed for each file transferred.

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Figure 26.13 Using the control connection

• Control Connection
• Created in same way as other application
  programs.
• Connection remains open during the entire
  process.
• Service type used by the IP is minimize delay,
  because this is an interactive connection between
  a user (human) and a server.
• User types commands and expects to receive
  responses without significant delay.
• Data connection
• Opened when data are ready to transfer. Closed
  when it is not needed.
• Service type used by IP is maximize throughput.

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Figure 26.14 Using the data connection

• Communication over control connection
• Uses ASCII character set.
• Each command or response is only one short line,
  so we need not worry about file format or file
  structure.
• Each line is terminated with a two-character
  (carriage return and line feed) end-of-line
  token.
• Communication over data connection
• Client must define the type to be transferred,
  the structure of the data, and the transmission
  mode.
• Heterogeneity is solved by file type, data
  structure, transmission mode.

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• File type
• ASCII file
• By default for transferring text files.
• Sender transforms the file from its own
  representation to ASCII characters
• Receiver transforms the ASCII characters to its
  own representation.
• EBCDIC file
• If one or both ends of connection use EBCDIC
  encoding (used in IBM computers) the file can be
  transferred using EBCDIC encoding.
• Image file
• Default format for transferring binary files
• File is sent as continuous streams of bits
  without any interpretation or encoding
• Used for compiled programs or images.

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• File is encoded in ASCII or EBCDIC, another
  attribute must be added to define the
  printability of the file.
• Nonprint
• Default format for transferring a text file.
• File contains no vertical specifications for
  printing.
• File cannot be printed without further processing
  because there are no characters to be interpreted
  for vertical movement of the print head.
• This format is used for files that will be stored
  and processed later.
• TELNET
• File contains ASCII vertical characters such as
  CR (carriage return), LF (line feed), NL (new
  line) and VT (vertical tab).
• File is printable after transfer.

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• Data structure
• FTP can transfer a file across the data
  connection using one of the following
  interpretations about structure of data
• File structure (default)
• File has no structure.
• Continuous stream of bytes
• Record structure
• File is divided into records (or structs in C).
• Used only with text files.
• Page structure
• File is divided into pages, with each page having
  a page number and a page header.
• Pages can be stored or accessed randomly or
  sequentially.

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• Transmission mode FTP can transfer a file across
  the data connection by using one of following
  three transmission modes
• Stream mode
• Default mode
• Data is delivered from FTP to TCP as a continuous
  stream of bytes TCP is responsible for chopping
  data into segments of appropriate size.
• End-of-file is closing of data connection by
  sender.
• If data is divided into records, each record will
  have 1-byte end-of-record (EOR) character, and
  the end of the file will have a 1-byte
  end-of-file (EOF) character.
• Block mode
• Data can be delivered from FTP to TCP in blocks.
• Block is preceded by 3-byte header.
• 1st byte is called the block descriptor next 2
  bytes defines the size of block in bytes.
• Compressed mode
• If file is big, data can be compressed.
• Commonly used compression method is run-length
  encoding.
• Consecutive appearance of a data unit are
  replaced by one occurrence and number of
  repetitions. Usually, Blanks in text file and
  null characters in binary file are compressed.

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Figure 26.15 File transfer

• Means of File Transfer
• Retrieving a file Copy file from server to
  client.
• Storing a file copy file from client to server.
• List of directory or file names is to be sent
  from the server to the client. Note that FTP
  treats a list of directory or file names as a
  file. It is sent over the data connection.

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Example 1

• Figure 26.16 (next slide) shows an example of how
  a file is stored.
• The control connection is created, and several
  control commands and responses are exchanged.
• Data are transferred record by record.
• A few commands and responses are exchanged to
  close the connection.

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Figure 26.16 Example 1
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Table 26.4 List of FTP commands in UNIX
Commands
!, , account, append, ascii, bell, binary, bye, case, cd, cdup, close, cr, delete, debug, dir, discount, form, get, glob, hash, help, lcd, ls, macdef, mdelete, mdir, mget, mkdir, mls, mode, mput, nmap, ntrans, open, prompt, proxy, sendport, put, pwd, quit, quote, recv, remotehelp, rename, reset, rmdir, runique, send, status, struct, sunique, tenex, trace, type, user, verbose,?
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Example 2
We show some of the user interface commands that
accomplish the same task as in Example 1. The
user input is shown in boldface. As shown below,
some of the commands are provided automatically
by the interface. The user receives a prompt and
provides only the arguments.
ftp challenger.atc.fhda.edu Connected to
challenger.atc.fhda.edu 220 Server ready Name
forouzan Password xxxxxxx ftp gt ls
/usr/user/report 200 OK 150 Opening ASCII
mode ........... ........... 226 transfer
complete ftp gt close 221 Goodbye ftp gt quit
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Example 3
We show an example of using anonymous public
access OR without an account FTP. We connect to
internic.net, where we assume there are some
public data available. Sometimes, anonymous as
user name and guest as password
ftp internic.net Connected to internic.net 220
Server ready Name anonymous 331 Guest login OK,
send "guest" as password Password guest ftp gt
pwd 257 '/' is current directory ftp gt ls 200
OK 150 Opening ASCII mode bin ... ftp gt close 221
Goodbye ftp gt quit